1. Field of the Invention
The invention relates to a method for partially or completely coating the surfaces of components produced from aluminum or its alloys with solders, fluxing agents or binders for brazing the components to one another, wherein solder, fluxing agents and binders are applied at high speed onto the surfaces provided for brazing, wherein heat is used for melting them.
2. Discussion of the Related Art
The German patent application with the Serial No. 197 44 743.1-45 describes a method of the aforementioned kind wherein the solder and the fluxing agent are applied in a powder form at high speed for a uniformly metered coating of the surface of metal components while the binder is applied in a liquid or at least pasty form in a limited way onto the surface of the metal components. In this known solution a coating that adheres well is obtained which makes possible further processing steps. For reasons of economic efficiency a high coating speed should be aimed for so that an integration into already existing processes is made possible. However, the limited binder application requires also a relatively large amount of binder for a sufficient adhesion of the fluxing agent and solder in powder form, relative to a surface, and, moreover, solvent emissions of the binder as well as waste material result with which disposal costs are incurred.
This is the starting point of the invention whose object it is to improve the aforementioned method such that for obtaining a good soldering result with a coating as uniform as possible the applied layer containing solder, fluxing agents and binders can be produced with the smallest possible quantities of these materials. This object is solved according to the invention in that in addition to the solder and fluxing agents in powder form the binder is also applied in powder form. This application of solder, fluxing agent and binder onto the surface of the components of aluminum and its alloys is carried out advantageously by an electrostatic powder coating process. For this purpose, two methods can be advantageously employed wherein, on the one hand, the triboelectric chargeability of the powder components or, on the other hand, the ion-depleted corona charge of the powder components can be used. These powder components of solder, fluxing agent and binder in powder form can be applied as individual components by means of a pistol onto the surface of the component. The application of the solder, fluxing agent and binder by means of a pistol can be carried out even when these materials have been previously transferred into the form of a co-extruded material and in this way a powder mixture of solder, fluxing agent and binder is formed. In this connection, the co-extruded material can be applied in an especially environmentally safe and economical way in an electrostatic fluidized chamber or in a fluidized bed.
The application in an electrostatic fluidized chamber or a fluidized bed can be performed also with solder, fluxing agent and binder in powder form when these materials are present as individual components in powder form. A further possibility of application in an electrostatic fluidized chamber or a fluidized bed resides in that the solder, fluxing agent and binder in powder form are applied in the form of a powder mixture.
In addition to the aforementioned application methods a magnetic brush is also suitable for applying the solder, fluxing agent, and binder in the form of a coextruded material onto the surface of a component of aluminum and its alloys.
The aforementioned process possibilities of powder coating by means of pistol, fluidized chamber or magnetic brush make possible coating speeds of up to 180 m/min. Moreover, the coating variants by means of electrostatic fluidized chamber or magnetic brush have the advantages of an especially uniform coating while, when using pistols, a slight pulsation of the powder flow can occur.
In this connection it is also possible that, in addition to the three aforementioned components of solder, fluxing agent and binder, also functional additives for wear protection and corrosion protection can be added which can also be applied in a powder form. The functional additives are, for example, carbides for wear protection and, for example, zinc for corrosion protection.
For forming a uniform layer of solder, fluxing agent, and binder, the powder application onto the surface of the component must be subjected to heat whose temperature range may be 80xc2x0 C. up to approximately 350xc2x0 C. This can be achieved, for example, in that the processing heat generated during the extrusion process in the component can be used for an application process which directly follows the extrusion process as the heat for melting of the layer of solder, fluxing agent and binder to be applied to the component. However, when before powder coating of the component the component has cooled, the heat for melting of the layer of solder, fluxing agent and binder applied in powder form is to be applied advantageously to the component directly before or even after the powder coating process.
For after treatment with respect to a surface sealing and optionally also smoothing of the melted layer of solder, fluxing agent, and binder, this layer can be supplied with heat in a further method step. This can be realized by heat radiation or also in the case of an additional smoothing by heated rolls which are pressed against the surface of the coated component. Optionally, within the context of the after treatment, a parting agent can also be applied onto the sealed layer in order to prevent adhesion during further processing, for example, by coiling thin-walled components. The parting agent can be, for example, oil. For controlling the powder application with respect to the layer thickness to be built up and its consistency as well as for controlling the heat supply, the after-treated melted layer of solder, fluxing agent and binder preferably passes through a measuring station which inputs data into a control circuit for controlling the coating process.
Downstream of the measuring station, the coated component advantageously passes through a cooling device with which these coated components can be brought to a temperature suitable for further processing, for example, by cutting, straightening, coiling, for their storage and the like.
For the brazing step, the application of, in particular, a very thin layer of solder powder, fluxing agent powder, and binder powder onto the component surface is sufficient, wherein the layer thickness is less than 30 xcexc, preferably 15 xcexcm. For this purpose, it is sufficient to apply an amount of binder powder onto the component surface for brazing which is 3 to 10 g/m2, preferably however 4 g/m2.
As the reaction mixture for performing the method, solder, fluxing agent and binder available in powder form are suitable. The powder particles are always present within a distribution range. For example, powder particles of a size of 3 to 50 xcexcm may be present in the distribution range. The average of the distribution of the powder particle size should be 3 to 30 xcexcm, preferably 10 to 15 xcexcm. The binder in powder form is advantageously a fine-grain organic binderxe2x80x94in the following referred to as clear lacquer powderxe2x80x94while the solder powder is in the form of aluminum-containing metal powder, in particular, a metal or alloy powder containing Alxe2x80x94Si, preferably Alxe2x80x94Si(7-40) alloy powder. However, it is also possible to use as a solder powder a silicon powder or zinc powder. The fluxing agent is advantageously a non-corrosive fluxing agent on the basis of metal fluorides, in particular, alkali metal fluoride and/or zinc fluoride.
For forming the reaction mixture in powder form the weight ratio of solder powder to the fluxing agent powder is also important. In this connection, the weight ratio of solder powder to fluxing agent powder is 1:1 to 1:3 wherein, however, preferably a weight ratio of 1:1 is provided. Furthermore, for the formation of the reaction mixture the percent by weight proportion of binder in the form of clear lacquer powder, relative to the total amount of powder of solder, fluxing agent and binder, is important, wherein preferably the binder contains 25% by weight clear lacquer powder, relative to the total weight of the powder.
With respect to the aforementioned coextrusion powder, it should be noted also that the formation of such a coextruded powder of solder, fluxing agent and binder is beneficial inasmuch as no separation of the components will occur. This co-extruded powder can be formed by grinding a granular material which is obtained by extrusion of a mixture of solder, fluxing agent and binder wherein the coextruded powder particles have a uniform particle size distribution.